Wavelength Division Multiplexing (WDM) is known in the optical communication field. In the wavelength division multiplexing, alight of two or more optical signals having different wavelengths is transmitted over an optical fiber and then demultiplexed by wavelength at the receiving end. This sometimes results in generating noises that affect optical signals having different wavelengths. Especially, when the gain of an optical fiber amplifier that amplifies a signal light has wavelength dependence, a variation is generated in the signal light intensity after the light is amplified. Therefore, there is a need for compensating for this variation.
To compensate for such a variation in the light intensity, it is known that a gain equalizer for an optical fiber amplifier is provided at the output side of the optical fiber amplifier. This gain equalizer uses a loss filter to reduce the wavelength dependence of the intensity of an amplified signal light and thereby smoothes the wavelength dependence.
A conventional loss filter amplifies a light using a gain medium such as a rare-earth doped optical fiber and, after that, filters the amplified light. The filtering characteristics of this loss filter depend on the final gain characteristics of the whole gain medium. If the gain characteristics of the whole gain medium have the wavelength dependence such as the one indicated by curve A in FIG. 11(a), the filtering characteristics of the loss filter are set so that the filtering characteristics have the wavelength dependence such as the one indicated by curve B in FIG. 11(b). Curve B is set so that its profile has a heavy loss in the part where the gain of curve A is relatively high. The light intensity of the light amplified by the gain medium has the wavelength dependence such as the one indicated by curve A. The loss filter, which partially reduces the part of the light intensity where it is relatively high, gives a light having the characteristics indicated by curve C in FIG. 11(c).
As described above, the filtering characteristics of the conventional loss filter are based on the wavelength dependence of the gain of the whole gain medium, and it is assumed that the wavelength dependence does not vary but remains static.
However, the wavelength dependence of an optical fiber or a gain medium changes over time. This means that a loss filter with fixed filtering characteristics, if used for compensating for the wavelength dependence, generates a variation in the light intensity as the time goes by. To configure an optical network, a ring structure or a mesh structure must be formed using optical fibers. In such an optical network, the route length may change according to how the route through which a light passes is switched. In this case, a variation is generated in the light intensity.
Therefore, there is a need for a gain equalizer capable of selectively controlling a specific wavelength of a light. This gain equalizer is essential especially for a high-speed, long-distance optical network.
A dynamic gain equalizer is proposed as a variable attenuator that equalizes the light intensity that varies according to the wavelength. The proposed dynamic gain equalizer uses the MEMS (micro electro mechanical systems) to perform dynamic gain equalization through the diffraction effect or the reflection direction. In the configuration of a MEMS where the diffraction effect is used, the micro-sized flat mirrors arranged in an array form are driven to build a square-wave-shaped irregular structure that forms a diffraction grid for dynamically controlling the light intensity. In the configuration where the reflection direction is used, the light intensity of each wavelength is dynamically controlled by changing the inclinations of micro-sized flat mirrors arranged in an array form.
FIG. 12 is a diagram showing the overview of a dynamic gain equalizer that uses the MEMS conventionally proposed.
In a dynamic gain equalizer 101, a light exiting an optical fiber 102 enters a diffraction grid 104 via a lens 103. The diffracted light diffracted by the diffraction grid 104 enters a MEMS 105 via the lens 103 for each wavelength. The MEMS 105 moves or inclines the micro-sized flat mirrors arranged in an array form to change the light intensity of the reflected light for each wavelength. The reflected light whose light intensity is changed by the MEMS 105 for each wavelength returns to the optical fiber 102 via the lens 103 and the diffraction grid 104. The MEMS 105, which controls the light intensity of the reflected light according to the wavelength dependence of the optical fiber or the gain medium, smoothes the wavelength dependence of the light intensity of the light that is returned to the optical fiber 102.
The dynamic gain equalizer, which uses an MEMS, requires as many MEMS-based micro-sized flat mirrors as there are wavelengths to be controlled and requires a resolution approximately equal to that of the light diffracted by the diffraction grid and the lens. A problem is that the mechanism becomes complicated in order to satisfy those requirements. Another problem is that a mechanical moving part sometimes causes a sticking that prevents the mirrors from being moved smoothly. These problems generate a problem in controllability and reliability.
A MEMS has no exit of light other than the optical path formed by the reflection of the micro-sized flat mirrors. A problem with the amount of light that remains after the light intensity is controlled by the MEMES (difference between incoming light and outgoing light) is that the light scatters in the MEMS and becomes noises or is that the light is converted to heat that increases the internal temperature. The problem of this scattered light or heat generation is serious when the dynamic gain equalizer is miniaturized or the optical signal capacity is increased.
To smooth the wavelength dependence of light intensity in a dynamic gain equalizer, it is required to monitor a change in the wavelength dependence of light intensity and, if a change is detected, to control the dynamic gain equalizer in order to compensate for that change. To monitor a change in the wavelength dependence of light intensity in a conventional dynamic gain equalizer that uses the MEMS, it is required to provide an optical spectrum analyzer with the same configuration as that of the dynamic gain equalizer outside the dynamic gain equalizer and to introduce a light branched from the light, which enters the dynamic gain equalizer, into the optical spectrum analyzer for use in measurement. One example of the optical spectrum analyzer has a configuration in which a photodiode array is provided instead of the MEMS.
This optical spectrum analyzer, though built in the same configuration as that of the dynamic gain equalizer, cannot share the configuration with the dynamic gain equalizer. Therefore, two configurations each with the configuration of the dynamic gain equalizer are required and, as a result, the device becomes large. In addition, the dynamic gain equalizer receives the light from which part of the light is branched off to the optical spectrum analyzer. Another problem here is that the light intensity of the light exiting from the dynamic gain equalizer is reduced by the amount of light branched off to the optical spectrum analyzer.
In view of the foregoing, it is an object of the present invention to solve the conventional problems, to provide a dynamic gain equalizer capable of selectively controlling specific wavelengths and equalizing the optical intensity for each wavelength without using a mechanism that has a mechanical moving part such as the MEMS, and thereby to increase controllability and reliability. It is another object of the present invention to solve the problem of light scattering or heat generation, which is caused by a remaining light, by compensating for it and to eliminate the need for providing an optical spectrum analyzer separately required for monitoring a light and the need for branching off the light for monitoring.